The present invention relates to a clamping device with variable clamping force, especially to a clamping device by employing force balance between restoring force of resilient body and centrifugal force of moving mass to provide variable clamping force at different rotational speed application of an optical disk drive.
Optical disk drives with higher transfer rate and faster access capability become more and more popular as the information storage demand is drastically increased. Historically, the access speed is increased from 1xc3x97 speed at early stage to 52xc3x97 speed (max) nowadays. In other word, the rotational speed of spindle motor is also increased from 200xcx9c500 RPM to more than 10,000 RPM. A drive operating at high-speed access will induce more vibration and centrifugal force on disk; therefore, the clamping device should guarantee sufficient clamping force to prevent the disk flip-flop away from turntable.
However, cost of the optical disk drive increases as aimed at providing larger constant clamping force for a traditional clamping device. Moreover, the tray motor exhausts more power and larger current during the loading and unloading process. Meanwhile, unpleasant noise possibly occurs and the disk may be dropped out of the tray if a large jerk happened by overcoming the so-called larger de-clamping force.
As shown in FIG. 1, a prior art of clamping device is comprised of a spindle motor 1a to rotate an optical disk 3a. A turntable 2a is to support the optical disk 3a thereon, thus maintaining suitable run out for a pick-up head to stably access information from disk. A rubber pad 20a is attached on the turntable 2a to provide sufficient friction at full speed.
Moreover, a clamping device is put upon the optical disk 3a to prevent it from dropping out. A base 4a and an annulus magnet 40a are the main components of the clamping device. A metal cover 41a is then fixed atop the annulus magnet 40a. On the other hand, an annulus yoke 21a with low carbon-concentration is mounted on the turntable 2a; a place axially aligned with the annulus magnet 40a, thus forming a constant air gap of a magnetic close-loop to firmly clamp the disk 3a. Alternatively, same effect will be achieved by mounting the magnet atop the turntable 2a and the annulus yoke 21a of low carbon-concentration atop the base 4a. 
Functionally, when the optical disk 3a is put on the tray (not shown) of an optical disk drive, the spindle motor 1a will then be activated to start up. If the built-in servo system detects the existence of a disk on the tray, lens of the pick-up head will be on focus to the tracks of a disk. The spindle motor 1a is accelerated to full speed after servo on. For the unloading process, the spindle motor 1a will be braked right before de-clamping the optical disk 3a and the tray will be moved out slowly if the user likes to take back the disk 3a and trigger the eject button.
Based upon the theory of Rotor Dynamics, the centrifugal force of a rotary body is proportional to eccentric amount (imbalance) and square of rotational speed. Therefore, the clamping device should exert sufficient attraction force in normal direction and the rubber pad 20a should possess large static friction coefficient to provide secure clamping force.
However, there are some drawbacks of prior art with constant clamping force listed as following:
(1) Most of the disks are produced with thickness 1.2 mm on the market, while the allowable thickness of a standard optical disk defined by the Red Book is ranged from 1.1 mm to 1.5 mm. The thicker of the disk, the weaker of the clamping force.
(2) The optical disk drive should be able to read CDDA, VCD, and DATA disks. Functionally, DATA disk is operated at full speed while the rest at lower speed. It seems not suitable for a clamping device to provide constant clamping force no matter what the angular velocity of the disk is.
Although larger clamping force can be achieved either through increasing the magnetic energy of magnet or reducing the air gap of magnetic loop, however, cost is indeed increased for adopting high-end magnet and larger clamping force makes the disk de-clamping more difficult.
Goal of the present invention is aimed at proposing a clamping device with variable clamping force. A novel clamping device can provide variable clamping force for optical disk drives to operate at different rotational speed. Physically, people have an intuition to apply smaller clamping force at lower speed while larger one at higher speed, thus providing stable operation and cost reduction opportunity.
To achieve above object, the present invention comprises a base, a resilient body with restoring property, an annulus magnet (or an annulus yoke) which can exert attraction force to a turntable of the optical disk drive, and a linking member which can change height of the annulus magnet (or the annulus yoke) relative to base according to the centrifugal force caused by variation of rotational speed of the optical disk drive. The centrifugal force is proportional to square of rotational speed such that the gap between the annulus magnet and the annulus yoke is accordingly adjustable. Variable clamping force is thus generated by variation of the gap.
The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawing, in which: